Remote control system



July 2, 1940.

C. S. SNAVELY REMOTE CONTROL SYSTEII Filed July 19. 1938 3 Shuts-Sheet lMd v TU N Na R w o m w \AW Q a m E a H figm S i July 2, 1940. c. s.SNAVELY 2,206,435

mums CONTROL SYSTEI Filed July 19, 1938 a Sheets-Sheet 2 INVENTOR I IClamzze .Szzaz/ely. '0 BY HIS ATTORNEY y 2, 1940. c. s. SNAVELY2,206,435

REMOTE CONTROL SYSTEJI Filed July 19, 1938 3 Sheets-Sheet 3 TBA B IN-NTOR Clare-11m HIS ATTORN EY Patented July 2, 1940 srATEs ran-rearoFFicE I N a 7 2,206,435 7 r r J .REMOTECONTROL SYSTEM Glarence S.sum-1y, Pittsburgh, Pa, asslgnor to The'Union Switch 82 Signal Company,S'wissvale, Tar, a corporation of Pennsylvania Application July 19;.1938, SerialN c. 219,978 :12 Claims; (01;. iii- 353 My inventionvrelates to remote control systoms, for the control and indication ofaflarge number of devices from a control-point,..and. {is

, particularly adapted. for use incentralized traf- "fice control.systems for railroads, of the type in whichtraflic governing-devices.alonga trackway are controlled. from a train dispatchers oifice,#and-theconditions of such devices and associatedldevices' .are indicated in.such office, by

the transmission'of code signals transmitted over relatively smallconductors and high electrostatic capacity. A iurther object is theprovision of. line'ci-rcuits in which induced voltages from external.sources will be neutralized to a large e:- tentiand will not interferewith the proper operatic-n of the system.

One-feature of my invention is the provision of suitable repeaterstations by means of which the code signals may be repeated.from'onecircuit to another as often as required, 7, g Other objects andfeatures of my invention will become apparent as the descriptionproceeds.

.It is to be understood that the centralized traffice control system.embodying my invention includes apparatus at the control station, ordispatchers omce, for generating difierent series of code impulsesarranged:iii-distinctive combinations, each such series constitutingacode message. for selecting and controlling apparatus ata particularremote station, and thateach re-1 mote station is provided withapparatus which is generally similar to that at the control o'flice and.which when initiated will operate through a cycle to transmit acombination of code impulses for indicating at the ofilce the stationoforigin andthe condition of devices at such station Myinvention is notrestricted to the use" offcod'e generating and receiving apparatus of aspecificform, but may be used with any'suit able system. In thespecificembddiment "of my invention herein illustrated, however, I have usedapparatus? for generating and receiving codes Which is similar'to thatof tl ie- Time code." so enjle of CentralizedTraflic Control asdisclosed and claimed in a copending application, Serial No;SiOOflBd-filed March 23, I932, by Lloyd V.

Lewis, for Remote control systems, which corresponds with British PatentNo. 415,787, issued September 3, 1934. The apparatus of my invention inthe specific form shown herein is an improvei-nent upon the line circuitportions of the apparatus of. the said Lewis application and. patent.

I. shall first describe one form ofapparatus embodying my invention, andshall then point out the novel features thereof in. claims.

In the accompanying drawings, Fig. 1 is a diagrammatic View illustratingtypical portions of. the line circuits of my invention, including theapparatus directly associated with the line cir-- cuits at the controloffice, at a repeater station,

and at a field station. Figs. 2 and 3 illustrate the local coding.apparatus at the control ofiice; and. at a field station, respectively,-this app'aratus being generally similar to the corresponding portions ofthe Lewis system. above referred to. Fig. 4. is a single line diagramwhichillustrates one manner in which the line circuits of my in ventionmay be arranged to control apparatus situated along an extensivestretch. of railway track. of which only the two end portions are shown,It is to be understood that. in Fig. 4, the apparatusidentified by thereference 0 corresponds to the ofice equipment of Figs. 1 and 2, thatidentified by the references R I, R'2,

Er -'3; eta, each corresponds to that of. a repeater station such as isshown in Fig. .1, and. that the apparatus identified by the referencesS-l, 'S-2, S-3 iand S--ii5, respectively, each corresponds to thestation equipment of Figs. 1. and 3, it is also to be understood that inFig. 4, each. station equipment is employed to control the track switchW and the associated. signals at one end of a passing siding by codestransmitted from the control ofiice and. to indicate the condition. or

the switch and. signals and the presence of trains on the adjacenttracks as explained in the Lewis application, and that the arrangementshown. is

-merely typical and may be varied as required. in

station to the ofiice.

- those stations more remote from the control of- The apparatus of myinvention is distinguished; by the employment of two normallyclosed linecircuits in place of the single line-circuitof the Lewis application.One oftheseisenergized by. battery at the omce end and is utilizedforthe transmission of control codes from the ofiice -to the severalfield stations, while the other is normally energized by battery at thedistant end,

and serves as a communication channel for the transmission of indicationcodes from any field Each line circuit has an auxiliary function, whichcomes into play when the other line circuit is in use, which is to lockout the transmitting apparatus at stations which have codes to transmit,so as to prevent interference with the station which is'transmitting acode. Furthermore, my line signals are characterized by reversals ofpolarity as distinguished from closed and open intervals. As will beapparent from Fig. l, the codes are delivered'to the line circuit by theoperation of pole-changer contacts, and are received by line relays R,each of which, it is to be understood, is of the two-position polarizedtype, so arranged that its contacts will remain in their last operatedposition when the relay is deenergized. Each line circuit used for codetransmission also includes at its battery end, a checking relay K of theneutral type, (a biased polar relay may also be used) each relay Kbecoming released to prevent further code transmission when thecorresponding line circuit is opened, but remaining picked up during theoperation of the associated pole changer during code transmission.

In order to simplify the disclosure and to enable the circuits to bemore readily understood, they have been arranged in the written circuitform, with similar circuits grouped together, but with relay contacts inmany instances at points on the drawings not adjacent the relay windingwhich operates them. In every such case I have applied to the contactthe reference character for the relay which operates it, and inaddition, a separate reference character, a, b, 0, etc., identifying thecontact. Furthermore, instead of showing the local source of current forenergizing the relays, I have shown only the terminals which aredesignated by the reference characters B and C, referred to the positiveor supply terminal and to the negative or common return terminal,respectively, of the source.

Relays having corresponding functions at the I office and stations areindicated by corresponding reference characters, the. oflice relaysbeing distinguished by the prefix O, in each case.

Referring to Fig. 1, the relay OM is the master relay of the ofiicecoding unit, and is energized only for the duration of a control codeand governs the transmission of such code, while relay OT is thetransmitting relay' which by the operation of its pole-changing contactsb and d delivers the control codes to the control line circult. ,Thefirst section of the control line circuit may betraced from theleft-hand or positive terminal of battery 9, at the office, back contact11 of relay OM, left-hand contact a of the indication line relay IR, theupper winding of the control line relay OR, line wire XI, the windingsof relay RI and contact a of relay KI at the repeater station, line wireX2, the lower winding of relay OR, contacts b of relays IR and OM, thewinding of relay OK, to the right-hand terminal of battery 9; Similarly,the-second section of the control line circuit may be traced frombattery II) atthe repeater station through the winding of relay KI andthe pole-changer contacts a and b' of relay RI, line wires X3 and X4,

end, ovenbackcontacts b and 720i the station master relay M; line wiresY3 and Y4, front con tact a of relay K2 and back contact a'of'a slowacting repeater relay K2P in' multiple, the circuit being completedthrough the windings of the repeating line relay R2. The polarity ofbattery I I is such that relay R2 "operates its contacts to the left asshown. h

The second section of the indication line circuit includes the batteryI2, the winding of relay K2,

and the pole-changer contacts a and b of relay R2 at the repeaterstation, line wires YI and Y2;

the indication line relay IR, back contact i-of re lay OM and frontcontact 11 ofrelay OK at the oilice. held to the left, as shown, andtherefore maintain normal polarity on the control line circuitj Thepurpose of the line battery is to maintain relays R2 and IRene'rgize'din their normal position when the equipment is at rest. When anindication codeis initiated,.however, relay M at the transmittingstation picks up and at its contacts I) and h disconnectsbattery I I andconnects a local battery I2 over the pole-cha'nger contacts b and d ofthe associated transmitted relay T to that portionof the indication linewhich. extends toward, the ofi'ice, this portion including the windingof a checking relay K at the station. It will be apparent from thedrawings that a failure of either section of the control line will causerelay OK to release and open, at its contact a, the local circuitscontrolled thereby,which' as hereinafter pointed out, will preventthedelivery of a control code to an open line,

points as desired, as indicated for example,iby

the specific arrangement illustrated in Fig. Referring now to Fig.2, the.coding unit therein illustrated is normally effective torespondselectively to indication codes delivered;by. the several fieldstations, and is rendered effective to deliver control' codes to thecontrolline a d- 9 si u a eo l e ste uch o 2 h The contacts of relay IR'are normally R lay; OK, at its contact b, at the same time opens therelay OT, so that the line remains steadily ener gized by current ofnormal polarity.

This terminates the code, and relays OLZ, OLP, OLB, OM and OLBP releasesuccessively. Relay OLBP opens the circuits for those relays of thecoding unit whichare energized, including relay OLI, and the latterrelay; upon releasing, closes its back contact I) to condition theequipment for the transmission of another code.

Although steps nine to thirteen of the code have for simplicity beendescribedas short steps,

it will be apparent fromthe drawings that any of these maybe prolongedby completing a holding circuit for relay OT from terminal B over one ofthe front contacts I of the stepping relays OI to O5.

:I shall now describe the operation of the station coding unit of Fig. 3in receiving the control code just described.

Relay R is, of course, operated by the code in the'same manner as relayOR and the various local relays controlled by relay R operate in thesame manner as the corresponding relays at the offlce on':each, step ofthe code, with certain exceptions which will now be pointed out. It isto be understood that the station apparatus is adapted to transmit codesas well as to receive them, the transmission of an indication code beinginitiated by picking up relay M over the circuit connected to terminal Bat contact 0 of a starting relay ST, but this circuit is held open atcontact b of relay LI or L2 when a control code is being received.Relays Ll and L2 pick up in unison with the oflice relays OLI and 0L2,at the beginning of the code. The indication line relay K, it will beremembered, was released by the opening of back contact 7 of the ofiicerelay OM. It follows that when relay 1 picks up on the first step, acircuit is completed from terminal B over its front contact 0 and. backcontact b of relay K to pick up relay E. Relay E is held energized forthe duration of the control code, and opens its back contact 11 in thecircuit for relay M. Since the pick-up circuits for relays M and OM arecontrolled independently, it may happen that an indication code will beinitiated by picking up thestation relay M at the same time that theoffice relay OM picks up, but if this occurs, relay M will be releasedon the first step by the operation of relay E and the transmission of aconflicting indication code by the station will be prevented, thestation equipment being conditioned to receive the control code from theoffice by the operation of relay E. Relays F, G and S are connected todifferent front contacts d of the stepping relays at the differentstations in accordance with the code combination characteristics of thestation. As shown in Fig. 3, these relays are connected to contacts d ofrelays 2, 3 and 4, respectively, so that this station as shown isresponsive to the code No. 234

(only. It follows that the station selector relay S will pick up only atthe selected station illustrated in Fig. 3 when the ofiice relay 23 1spicks up, butfor any different code, one or more of the codesteps-Z, 3or 4 will be short and the selector relay S having the code combinationNo; 234 will not pick'iip. At theselectedstation,-

a delivery relay D is picked up in place of the;

have been omitted from the drawings since they have no bearing on thepresent invention.

Ishall next assume that the, apparatus has" been restored to normal andthatan indication code is to be transmitted from the station =of Fig. 3.ii

""I'he indication code is'initiated bypicklng up the starting relay STby means not shown,Feither in response to. a change in condition of oneof the station devices having indications to transmit,

or following the receipt of a controlcode at the station. As alreadyexplained, relay ST at its contact 0 closes the pick-upcircuit for relayM, which becomes energized and'disconnects the more remote stations fromthe indication line,

and completes a circuit from the line battery 1-2 through relay K for.temporarily maintaining relay R2 energized in its normal direction;Relays T and K now pick up;'and relay T reverses the polarity'of theindication linefor the first step repeater station, which in turnreverses the indication line relay IR at the ofllce. Relay IR reversesthe polarity of the control line, so that of the indication code,reversing relay R2 at the relays OR and RI reverse, and relay R!reverses the control-line relay R at the transmitting station.

- When the relay R at each station is reversed,-

the associated relays LI, 0!, L2, LP, LB and LBP pick up, at the sametime relay OR picks up the correspondingoflice relays as in'the case ofthe control code already described. When relay- LBP at thetransmitting'station picks up, a circuit is completed from terminal Bover its front contact 1, front contact a of relay K to pick up relayKP, which relay 'is held energized for the duration'of the code over itsown front contact a and back contact g of relay E, provided relay-Eremains deenergized. Relay KP by the oper-- ation of. its contact 2)places the local relays of Fig. 3 at the sending station under thecontrol ofcontact'e of relay T in place of contact a of the line relayR, for'the remainder of the indica- 1 "r- =50 out that the station code.i

It has already :been pointed tion relay ill will pick up if relays OMand M should happen to pick up at thesame time, in which case the ofiicewill take:precedence over the static-n. Assuming, that this occur, itwill be seenfrom Fig, 2 that relay OM does not.

can not pickup later to interferewith the indication code, becauseitspick-up circuit will be open at back contact --b --of relay OLI or0L2- controlled by relay OR until the code is completed. No'other fieldstation can interfere with the indication code after it is started, forthe reason that the pick-up circuit foreachrelay M is similarly heldopen by relay Ill or L2'controlled by the'corresponding relay R, whilethe relay M at the sending station only, energized over its stickcircuit;

' It may happen, -however, that two or. more. stationsstart-transmitting at the identical moer ment so that the stick.circuits for more than. one relay M are completed.

is held In this case, it will" be clear from Fig. 1 that the portion ofthe indication line circuit extendingifrom the transmitting stationnearer the officeto the more re-' mote, stations will be open at;back-,contactsi'b 75:

It e t relayM; 'at'ithe: station-nearer the ofilce. or: :at-reontact Id:of relay K2, iliithere; is a: repeaterihetween such n'earerstation andthe more remote stations; Relay: K "at. each more remote stationLoonsequentlyzwlll release and complete at-"its contact 12 a pick-upcircuit for relay .E at such station, and each such relay E at its .backcontacts 12 and cuwill open the circuits for relays M? and KP;respectively, atthecorresponding statiomto restore the apparatus to thereceiving condition. It will be clear therefore that but one: stationcan transmit a code at a time.

As above pointed out, the control of the step ping relays and. of relayLl, L2, etc., is trans ferred from the line relay R to relay '1 at thetransmitting station by relay-KP when relay LE1? picksup. 1 Relay LBP'also opens the pickup cir cult for relay T-at its back contact use thatrelay '1' releases to terminate the first stepof theindi cation codeshortly after relay KP picks up, re storing the polarity of the linecircuits to normal and ELISO'PiCKlRg up relays'z and L2; Relay lreleases, and closes the pick-up circuit for relay '1, but that relay isprevented from picking up because terminal Bis now connected to itslefthand terminal over a circuit including back contact b of relay F.Relay Ll is now deenergized, and'when it releases, it closes a circuitat its back contact 2 to pick up relay F, permittlngrelay T to pick upto terminate the second step. This action, it will be seen, is similarto that already described in connection with the transmission or" acontrol code by the equipment of Fig. 2. The action on the remainingsteps is also similar, that is, relays G and S'plck up on the third andfourth steps,jinresponse to the release of relays L2 and L1respectively, and control the holding circuits for relay T to prolongthese steps of the "code, while on the remaining steps of the code relay''1' is-*controlle'd only by its pick-41p circuit includin back contactsof the steppingrelays I, 3, and 1 so that these are short steps, in thecode described. Relay l picks up on the ninth step next inorder ai'terrelay 8, and energizes relay CB. to

prepare a circuit for relay 2, and relays 2 to l, inclusive, areoperated'on steps nine to fifteen, respectively, relaylfi beingoperated, in place of relay 8 when the indication line becomes steadilyenergizedby' current of normal polarity at the fend-0t the code. Relayit at its back contact deen'jer'gizes relay M, and also picks up arelayVP which at its irontcontact c connects terminal B to the l ft-handterminal of relay '1 to'prevent further operation of that relay, andtherefore relays M, L2, LP,LB, LBP, Ll release successively.The-releas'e'ofrel'ay M places the indication line under the control ofbattery I I, re'storing'the line circuits to normal and releasing relayK. Relay LBP releases the remaining local relays, as in the case "of theomce eduipment' already described.

it is believed that the manner in which this in. dicatlon code isreceived at the office in response to the-czlperation of the line relayOR will be apparent without further description in view of the detaileddescription of the operation of that equipment already given, and thatit will be clear that the selector relay 2345' will be operatedselectively in response to the transmitted code No.

7g -'Returning now to Fig.1, it is believed that inviewpifthekroregoing:description itis clear'. that additional fieldstations'mayj be included in. series in the line circuit, to any'extentdesired,-up to the maximum capacity: of the system,and that repeaterstations maybe added in series at. any point, in" accordance with thepractical requirements. of a particular installatiomwithout changing themode'of" operation: in any way, and that my apparatus is adapted for useunder a wide va: riety of conditionsand is particularly adapted toprovide reliable transmission over long distances and under adverseconditions. In practice, the most remote station may be several hundredmiles from the dispatchers office, and the line conductors maybe eitheropen wires or relatively small conductors enclosed in cable, withoutmaterially impairing theiquality of transmission as compared witharshort line. i

Certain features of Fig. l will-now be explained. It will be noted thatrelay K2 atthe repeater station at its front contact i controls. a slow'acting repeater relay-KER When the indication line is opened by relayOM, at the beginning of a control code, releasing relay K2, relay K21becomes deenergized and after a brief time interval releases toreestablish the continuity of the remote section of the indication lineat itsback contact a. This results in maintaining the repeater stationline relay R2 energized in its normal direction, so that when backcontact 7' of relay OM closes at the end of the control code, relay IRat the office,

will immediately become energized in the normal direction to supplycurrent of normal polarity to the control line over its contacts a andb." Other wise' it might occur that an extra impulse of reverse polaritywould be transmitted over the control line at the end of acontrol code.

The purpose of the office relay OK'in thecontrol line is toprevent'coding in case the control line becomes open. In case the lineis in a cable, the transient charging currents produced by the repeatedreversal of the line battery 9 by the polechanger contacts of relay OTmay be sufficient to operate line relay OR, even though the line is openat its distant end. Since relay OR. is ofthe the local coding'circuitsat its contact a and thereby prevent further transmission until the lineis restored. I

One advantage of the transmission system of the present application isthatthe line relays are held in their operated positions substantiallycontinuously by the current delivered to the line by the transmitter,and are therefore less subject to interference than when the codingis'effected by merely opening and closing the line, and the line relaysare arranged with balanced windings so as to be immune to a largedegree'to induced currents due to foreign sources. A further advantageis that the so-called bias distortion of the code signals is avoided.This term refers to a lack of symmetry in the waveform of the receivedcode signals which may exist when the signals are produced by openingand closing along cable line circuit, or in case appreciable leakage ofline current exists due to weatherconditions. As is well known, thetransmissionof code signals over extremely long distances and underadverse conditions can best be effected by merely pole-changing the linecurrent, as employed in the system of the present application.Furthermore, since each line circuit of the present system is used fortransmission in but one direction only, repeaters may be added withoutcomplication, as has already been pointed out, and the line circuits maybe controlled by a plurality of independent transmitters withoutinterference even though the transmitting stations are widely separated.

Although I have herein shown and described only one form of apparatusembodying my invention, it is understood that various changes andmodifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In a remote control system, an ofllce and a plurality of stations, acontrol line and an indication line connecting the offlce and stations,a code transmitter at the ofiice effective when initiatedto transmit amultiple impulse control code over said control line, a code transmitterat each station effective when initiated to transmit a multiple impulseindication code over said indication line, a repeating relay at theoifice having its winding included in said indication line and itscontacts in said control line for repeating over the control line eachindication code transmitted over the indication line, code responsivemeans at the office and at each station responsive to codes transmittedover the control line, and lock-out means associated with eachcode-responsive means for preventing the operation of the associatedcode transmitter as long as a code' is being transmitted over thecontrol line by another transmitter.

2. In a remote control system, a transmitting station and a receivingstation, a normally energized line circuit including a source ofcurrent, and a pole-changer at the transmitting station; a neutral relayin said line circuit between said source and said pole-changer, atwo-position polar line relay at the receiving station controlled inaccordance with the polarity of the current supplied to said linecircuit, means at the transmitting station effective only when saidneutral relay is picked up by current delivered to said line circuitfrom said source for operating said pole-changer to reverse the polarityof the current supplied by said source at time-spaced in' tervals ofvariable length to operate said line relay in accordance withdifferentcodes, and code receiving means controlled by said line relayand selectively responsive to the relative lengths of said intervals insaid codes.

3. In a remote'control system, a transmitting station and a receivingstation, a normally energized line circuit including a source of currentand a pole-changer at the transmitting station; a neutral relay in saidline circuit between said source and said pole-changer, a two-positionpolarline relay at the receiving station controlled in' accordance withthe polarity of the current supplied to said line circuit, means at thetransmitting station eifective only when said neutral relay is picked upby current delivered to said line circuit from said source for operatingsaid pole-changer to reverse the polarity of the current supplied bysaid source at time-spaced intervals of variable length to operate saidline relay in accordance with different 'codea'codc 'receiving'meanscontrolled by. said line relay and selectively responsive to therelative lengths of said intervals in said codes, and means at saidreceiving station for opening said line circuit to at times prevent theoperation of said polechanger. 1

4. In a remote control system, a control station, a repeater station anda remote station, a normally energized control line having'aflrstportion including a source of current, a' first neutral relay and apole-changer at the control station, and also including a two-positionpolar line relay at the repeater station, said line having a secondportion-including a source of current, a second neutralirelay and apole-changer controlled'by the line relay at said repeater sta-' tionandalso including'a two position polar line relay at the remote station;means effective only when the first neutralrelay is'picked up foroperating the pole-changer at the control station to periodicallyreverse the polarity of the current supplied to said first section tooperate said line relays in accordance with different codes, andmeans'controlled by the second neutral relay for opening the firstsection of the line circuit to prevent the operation of the pole-changeratthe control station in the event the second section of-the linecircuit is opened.

5. In a remote control system, a control sta tion and a remote station,a line circuit including a source of current and a polarized'llne' relayat the control station, a second polarized line relay at the remotestation controlled in accordance with the relative polarity of.energization of said line circuit, a master relay at the con trolstation, a first code transmitter at the con trol station controlled,from a remote point and effective only when said roaster relay'is'd'eenergized to deliver codes of time spaced impulses of alternatelyopposite polarity; andof variable length to said line circuit, a secondcode transmitter at-the control station efiective only when saidmasterrelay is energized to deliver different codes -ofytime spaced'impulsesof alternately opposite polarity and of variable length to said linecircuit, code responsive means at each station selectively; responsiveto the relative lengths of the impulses of the codes received by saidlinerelays, and starting means at the control station for energizingsaid master relay 'effectivegonly when saidfirst code transmitter is notin operation. i

6. In a remote control systerrna control 'sta tion and a remote station;a normally closed line circuit including a source of current and apolarized linerelayat'the control station, a'sec- 0nd polarized linerelay at the remote station controlled in accordance with therelative-po larity of energization'of said line circuit, amaster relayat the control station, a first code transmitter at the control stationcontrolled from a remote point and effective only when said master relayis deenergized to deliver codes of time spaced impulses of alternatelyopposite pc larity-and' of variable length to said line circuit, asecond code transmitter at the control station eifective only'when'saidmaster relay is'energlzed to deliver different codes of time spacedimpulses of alternately opposite polarity and of variable length to saidline circuit, code responsive means at each station selectivelyresponsive to the rela'-' tive lengths of the impulses of the codesreceived by said line relays, starting means 'at the control station forenergizing said master relaycifective only when said first codetransmitter is not in operation, and means for preventing the operationof either code transmitter when said line circuit is open.

'7. In a remote control system, an ofiice and a station, a normallyclosed line circuit including at the oflice a single current source anda first two-position polar line relay, a second two-position polar linerelay at said station controlled in accordance with the relativepolarity of the current supplied to said line circuit, a first and asecond pole-changer at the office each effective when operated toreverse the polarity of the line current from said source at time spacedintervals to produce a code, code receivers at the ofiice and stationeach selectively responsive in accordance with the relative time spacingof successive operations of the associated line relay, means at theofice for operating the first pole-changer repeatedly at selected timeintervals of difierent lengths to deliver a code to the line circuit towhich the station code receiver is responsive, and means at the stationfor operating the second pole-changer repeatedly at selected timeintervals of different lengths to deliver a code to the line circuit towhich the office code receiver is responsive.

8. In a remote control system, a control line including a plurality ofnormally closed circuit portions, each including a two-position polarline relay at one end and a pole-changer and a source of current at theother end, a neutral relay in each portion between the pole-changer andsource of current, code transmitting means at a control station forcontrolling the pole-changer for the first portion to deliver a code oftime spaced reversals of current to said line, means controlled by theline relay for each except the last portion for controlling thepole-changer for the next succeeding portion to repeat said code, coderesponsive means controlled by the line relay for the last portion, andmeans including said neutral relays effective to stop the operation ofthe transmitting means in the event any one of said circuit portionsbecomes opened.

9. In a remote control system, two stations connected by a normallyclosed line circuit including a source of current at one station, apolechanger at one station efiective when actuated to reverse thepolarity of the current delivered to said line circuit, a series ofstepping relays, means controlled by said pole-changer for operatingsaid stepping relays one at a time in order, one for each actuation ofsaid pole-changer, means responsive to each successive operation of astepping relay for actuating said pole-changer to thereby deliver aseries of relatively short impulses of alternately opposite polarity tosaid line circuit, means at said one station controlled in accordancewith a code for delaying the operation of the pole-changer when selectedstepping relays are operated to prolong selected impulses of saidseries, and code responsive means at the other station selectivelyresponsive to the relative lengths of the impulses of said seriesirrespective of their polarity.

10. In a remote control system, two stations connected by a normallyclosed line circuit including a source of current at one station, a polechanger at one station effective when actuated to reverse the polarityof the current delivered to said line circuit, a series of steppingrelays,

means controlled by said pole-changer for operating said stepping relaysone at a time in order, one for each actuation of said pole-changer,means for actuating said pole-changer in response to each successiveoperation of a stepping relay to deliver a series of impulses ofalternately opposite polarity to said line circuit, time con" trolledmeans for delaying the actuation of said pole-changer to prolongselected impulses of the series, means at said one station forselectively controlling said time controlled means in accordance with acode, and code responsive means at the other station selectivelyresponsive to the relative lengths of the impulses ofsaid seriesirrespective of their polarity.

11. In a remote control system, two stations connected by a first and asecond line circuit, a series of stepping relays at each station, meansfor operating the relays of each series sequentially when impulses ofalternately opposite polarity are transmitted over said first linecircuit, a code transmitting relay at one station for transmittingimpulses of alternately opposite polarity over said first line circuit,a code transmitter at the other station for transmitting impulses ofalter nately opposite polarity over said second line circuit, means atsaid one station for retransmitting over said first line circuit theimpulses re ceived over said second line circuit means at each stationcontrolled by the stepping relays at that station for at times operatingthe associated code transmitter and for governing the relative lengthsof the impulses transmitted over the corresponding line circuit to theother station in accordance with a code, code responsive meanscontrolled by each series of stepping relays selectively responsive tothe relative lengths of the impulses of the code delivered by the otherstation, independent means for initiating the operation of said codetransmitters, and means at one station for discontinuing the operationof the code transmitter at that station in the event the operation ofthe other transmitter has been initiated.

12. In a remote control system, an office and a plurality of stations,line wires connecting the ofiice and stations to provide a control linechannel and an indication line channel, a code transmitter at the ofliceeffective when initiated to transmit a multiple impulse control code tothe stations over the control line channel, a code transmitter at eachstation effective when initiated to transmit a multiple impulseindication code to the ofiice over the indication line channel, arepeating relay controlled over the indication line channel havingcontacts in the control line channel for repeating over the control linechannel each indication code transmitted over the indication linechannel, coderesponsive means at

